Vesicular stomatitis virus (VSV) is a widely studied prototype negative strand virus (NSV) that is currently being developed as a vaccine vector. Although laboratory-adapted strains of VSV are generally nonpathogenic in humans and nonhuman primates, the potential for VSV vectors to cause disease in humans has been addressed by the development of vectors that are attenuated for virus growth. This has the undesired effect of reducing the level of antigen expression and reducing the subsequent immune response. This project tests an alternative strategy for attenuating VSV vaccines. Instead of compromising the ability of the virus to replicate, this strategy involves introducing mutations that reduce the ability of the virus to suppress host innate immune responses. Such mutations are designed to attenuate viral pathogenicity by protecting the central nervous system (CMS) from virus infection. The same mutations enhance the induction of adaptive immune responses, due to greater stimulation of innate immune responses. The goal of this project is to determine the mechanisms by which VSV and other NSV vectors that express HIV-1 antigens induce innate and adaptive immune responses. Specific Aim 1 is to determine the role of different dendritic cell (DC) subsets in the immune response to NSV vectors that express HIV-1 antigens. The ability of HIV-1 vaccine vectors to activate myeloid DC (mDC) versus plasmacytoid DC (pDC) in vitro and their dependence on TLR7 will be determined. These experiments will test the hypothesis that mDC and pDC cooperate to enhance antigen presentation to T cells in vitro and in vivo. Specific Aim 2 is to determine the ability of VSV vectors to prime antibody and CTL responses against HIV-1 antigens. These experiments will compare VSV vectors in their ability to induce primary cellular and humoral responses against HIV-1 antigens. VSV vectors will also be tested for their ability to prime immune responses against HIV-1 antigens following boosting with rabies virus or Newcastle disease virus vectors from projects 1 and 2. The combination of vectors that is the most potent elicitor of anti-HIV-1 immune responses in Aim 2 will be tested for its ability to elicit antibody and CTL responses against SIV in nonhuman primates. These experiments will establish general principles for live virus vaccine vectors that are defective in their ability to inhibit host antiviral responses, and should also contribute to the development of novel vaccines based on VSV and other NSV vectors. RELEVANCE (See instructions): Vesicular stomatitis virus (VSV) is an animal virus that has been studied for many years as a prototype for many important human viral pathogens. More recently, genetically modified VSVs are being developed as vaccines against human diseases. The goal of this project is to determine how VSV stimulates immune responses and use this information to develop new candidate vaccines against HIV. This will provide fundamental new information on how viruses induce immune responses in their hosts, and should contribute to development of novel vaccines against HIV and other infectious agents for which vaccines are needed.